How long do solar batteries last? What the warranty says

The federal energy department's battery guidance gives no figure. It says to "Consider the expected lifetime of the battery which can be measured by" years, "cycles (the number of times it is charged and discharged)", "throughput (the amount of energy stored and delivered)" or a combination — then adds that "The savings you make from a battery may not be enough to pay for the cost of the battery within its warrantied lifetime."

A battery is sold on a headline number of years, but years are only one of the three measures, and the other two usually bind first.

One claim worth clearing away: being on the Clean Energy Council's approved list does not guarantee a ten-year or 70% warranty. The CEC's battery terms and conditions require a manufacturer to supply warranty terms, keep them consistent with Australian Consumer Law, and have an Australian entity with an ABN responsible for honouring them. They set no minimum term and no minimum capacity retention.

Every warranty we have read promises the battery will still hold some proportion of its rated capacity at the end of the term. That proportion is not standard.

Twelve of the sixteen batteries we review promise 70%. Tesla is typical: 70% of 13.5 kWh at ten years, about 9.5 kWh of usable storage on the last day of cover. The outliers run both ways. sonnen promises 80%, but measures it against nominal rather than usable capacity. SAJ promises 60%, and Redback 55% of nominal.

The sixteenth is the one to watch. AlphaESS publishes a ten-year, 70% warranty, but the Australian documents we checked do not list the BAT-9.3S module in the review, so its retention and throughput terms need written confirmation before purchase rather than being assumed from the brand.

Check which capacity the percentage is measured against before comparing two numbers: a higher percentage of nominal capacity is not automatically a stronger promise than a lower percentage of usable capacity.

Thirteen of the sixteen batteries also cap the total energy that may pass through them. Reach the cap and the performance promise ends, whatever the calendar says. The ceilings we recorded run from under 20 MWh to 60 MWh. The other three measure it differently or not at all: sonnen counts 10,000 complete recharge cycles instead of megawatt-hours, neither warranty covering the Redback states a numerical ceiling, and the AlphaESS documents do not name the module in our review at all.

The important detail is what the ceiling applies to. Tesla gives "Unlimited cycles" for solar self-consumption, time-based control and backup, and applies a "37.8 MWh of aggregate throughput" limit to "any application not listed above, or any combination of applications that includes one not listed above". Sungrow and Neovolt draw the same line in their own terms: ordinary household use is uncapped, other use is not.

That carve-out is not a market standard, though, and it is worth checking rather than assuming. SolaX splits the same two categories but caps both — 6,000 cycles for daily self-consumption and backup, and a 17.9 MWh limit for other applications — so even ordinary household use has a ceiling.

An illustration using Tesla's own figures, to make the units comparable: 37.8 MWh across a 13.5 kWh battery is about 2,800 full charge-and-discharge cycles, which at one full cycle every day would be reached in roughly seven and a half years. Most households do not cycle a battery fully every day, and the ceiling is a warranty boundary rather than the point at which a battery stops working. It is not a prediction about any household.

The ten-year headline is conditional in a way the sales material rarely reflects. The most common condition is an internet connection, because manufacturers use it to push firmware.

Tesla's warranty says that where a Powerwall "is not connected to the Internet for an extended period, or has not been registered", it may not honour the full term, and that "we will always honor the warranty for at least four years". FranklinWH is more specific: disconnected "for more than 90 consecutive days" without reconnection and "the warranty period will automatically be reduced to a four-year limited warranty", and a system not registered at installation gets four years from the start. Sungrow can fall to four years where monitoring records are incomplete; Sigenergy and LG can fall to five.

Temperature is the other recurring condition. Several warranties specify an operating range, and Sungrow goes further by setting an annual temperature profile the site must stay within for the performance cover to apply.

One rule sits outside the warranty but is tied to it: the Clean Energy Regulator states that a battery claiming certificates "should remain at the property for the duration of the warranty period or the end of 2030, whichever is later".

This is where the throughput ceiling and virtual power plants meet, and where most confident claims online are wrong in one direction or the other.

Fourteen of the sixteen warranties we read do not mention VPPs at all. They divide use into named household applications and everything else, which leaves VPP dispatch in the uncapped category for some products and the capped one for others, depending on the wording. No Australian regulator has published a general rule either way: the Clean Energy Regulator's VPP guidance only lists "understanding the impact of battery discharge rates on your system's warranty" as something to research before joining.

The clearest regulator warning concerns a different arrangement. The AEMC's analysis of residential battery profitability finds wholesale price pass-through plans can drive "potentially up to twice as much" cycling as a time-of-use tariff, which "can also reduce battery warranty due to exceeding cycling or throughput limits". The same analysis treats VPP participation more favourably, as a way to capture value "without the risk of being exposed to peaky, volatile prices".

The honest position is neither that a VPP voids your warranty nor that VPP use is always covered: the throughput clause governs, the wording differs by manufacturer, and a written answer about your intended program beats either generalisation. What a VPP is and whether to join one covers the control terms alongside this.

Two processes run at once: a battery ages with time whatever you do with it, and it ages with use. CSIRO's Batteries for Homes describes manufacturers deriving a fade rate from cycling tests, then using it to estimate "calendar life: the number of years before the battery's capacity falls to 80% of its original value".

Depth of discharge is the lever a household controls. CSIRO notes a shallowly cycled battery delivers far more total charge over its life than the same battery run from empty to full, and its summary of manufacturer claims for lithium chemistries spans 3 to 15 years and 3,000 to 6,000 cycles — a range, not a figure. Heat is the other factor: "Extreme ambient temperatures should be avoided, because low temperatures decrease battery capacity and high temperatures shorten battery life."

One risk appears on no specification sheet. Six years of independent testing funded by ARENA and run by ITP Renewables found more failures than expected, and noted that "Some manufacturers have become insolvent during the trial period, highlighting the importance of choosing manufacturers who will be likely to endure any issues and honour warranties where required". A twelve-year warranty is worth what the company behind it is worth in year eleven.

Ask for these in writing, from the manufacturer's current warranty document rather than a brochure:

• the term, and whether it runs from installation or from manufacture;
• the capacity floor, and whether it is measured against nominal or usable capacity;
• the throughput or cycle ceiling, and which uses it applies to;
• whether your intended VPP or trading arrangement sits inside the named household uses;
• the internet, registration and temperature conditions, and the reduced term if they are not met;
• who handles a claim, and whether diagnosis, removal, reinstallation and freight are covered;
• the document's version and effective date, so you can tell later whether the terms changed.

Every battery we review records these against the manufacturer's own document, with the version and the date we checked it. If you are also weighing the purchase price, the federal battery rebate guide sets out the current discount and its conditions.